1. Field of the Invention
The present invention relates generally to a mobile telecommunication system, and in particular, to a cell search apparatus and method in an asynchronous base station (BS) and a mobile station (MS).
2. Description of the Related Art
The UMTS (Universal Mobile Telecommunication System) is an asynchronous BS system in which an inter-BS operation is asynchronously done. Each constituent BS in the system is assigned to a unique cell specific code for identification. If the UMTS includes 512 cells (i.e. 512 BSs) for instance, 512 cell specific codes are assigned to the respective 512 BSs. To search for its serving BS, an MS should search the 512 BSs by checking the phases of their cell specific codes, consuming a large amount of time. To overcome the inefficiency of the conventional cell search algorithm, a multi-step cell search algorithm has been explored. For implementation of the multi-step cell search algorithm, the BSs in the UMTS are divided into a predetermined number of groups (e.g., 32 or 64 groups), each BS group being assigned to a different group specific code. Thus, for 512 base stations, each BS group is comprised of 16 BSs. Each of the 16 BSs is assigned to a different broadcasting channel (BCH) spreading code (cell specific code) so that the MS can search for its serving BS. This multi-step cell search algorithm includes the steps of (1) receiving a primary synchronization channel (P-SCH) signal from a serving BS and synchronizing to the slot time of a slot received with the highest power; (2) receiving a secondary synchronization channel (S-SCH) signal from the BS while the MS is synchronized with the slot time and detecting frame synchronization and the group specific code of the BS; and (3) searching for the cell specific code of the BS by means of a BCH received from the BS based on the frame synchronization and the BS group specific code.
The multi-step cell search algorithm will be described in detail referring to FIGS. 1, 2, and 3.
FIG. 1 illustrates a conventional UMTS channel structure. Along the time axis of a channel signal transmitted by a BS, one UMTS slot has 2,560 chips and 16 slots form one frame. Thus, one frame has 40,960 chips.
A P-SCH is the same code for each BS in the UMTS. It is transmitted with a synchronization code #0, SC 0, for a tenth of the each slot period, that is, for 256 chips. Then, an MS receives the P-SCH and synchronizes with a BS slot time (the first cell search step).
Along with the P-SCH, the BS transmits its BS group specific code on an S-SCH. The BS group specific code is a 16-symbol codeword. The 16 symbols corresponds to 16 synchronization codes selected from 1st to 17th synchronization codes SC1 to SC17 and each symbol is mapped in one of the 16 slots of one frame. The slot-time synchronized MS acquires the group specific code and frame synchronization of the serving BS from the received S-SCH (the second cell search step).
A BCH message signal transmitted along with the S-SCH signal is comprised of pilot and data symbols in each slot. The BCH message signal is spread with a cell specific code (a BS specific code), one of the 32 scrambling (cell specific code) code which mapped with the 16-symbol codeword, synchronization codes SC1 to SC16. The MS despreads the BCH signal with the 32 scrambling codes and selects a cell specific code with a maximum correlation as the BS specific code (the third cell search step).
The third cell search step will be described in detail referring to FIG. 2.
FIG. 2 is a block diagram of a conventional cell search apparatus to implement the third cell search step. Referring to FIG. 2, the MS, frame-synchronized with the BS and informed of the BS group in the second cell search step, receives the BCH signal at 1st to 32th correlators 211 to 223 in the cell search apparatus. Each BS group in the UMTS has 32 cell specific codes and a BCH signal is spread with one of the 32 scrambling code (cell specific code) which mapped with the 16-symbol codeword, synchronization codes SC1 to SC16. That is why the MS has 32 correlators to despread the BCH signal. The 1st correlator 211 calculates a correlation of the BCH signal in an auto-correlation function for a 1st cell specific code (scrambling code), a 2nd BCH correlator 215 calculates a correlation of the BCH signal in an auto-correlation function for a 2nd cell specific code (scrambling code), a 31th BCH correlator 219 calculates a correlation of the BCH signal in an auto-correlation function for a 31th cell specific code (scrambling code), and a 32th BCH correlator 223 calculates a correlation of the BCH signal in an auto-correlation function for a 16th cell specific code (scrambling code). Here, 3rd to 30th BCH correlators are not shown for convenience sake, but each unshown correlator calculates a correlation of the BCH signal in an auto-correlation function for the 3rd to 30th synchronization code. Then, the correlations are accumulated in 1st to 32th BCH accumulators 213 to 225. That is, the 1st BCH accumulator 213 accumulates the correlation received from the 1st BCH correlator 211, the 2nd BCH accumulator 217 the correlation received from the 2nd BCH correlator 211, the 31th BCH accumulator 221 the correlation received from the 31th BCH correlator 219, and the 32th BCH accumulator 225 the correlation received from the 32th BCH correlator 223. Though not shown, 3rd to 30th BCH accumulators accumulate the correlations received from the 3rd to 30th BCH correlators, respectively. A maximum BCH correlation detector 227 detects a maximum of the BCH accumulator outputs received from each BCH accumulator, to determine the BS specific code.
To detect its serving BS, as described above, the MS should perform the first to third cell search steps sequentially. This cell search method has the distinctive problems that (1) complex additions involved in the multi-step cell search consume a large amount of time and especially 16×256 complex calculations required to detect correlations of a 256-chip sequence with respect to 16 second synchronization codes take a long time and increase hardware load; (2) the requirement of 32 correlators corresponding to the BS specific codes for the third cell search step increases hardware complexity; and (3) many computations caused by concurrent parallel operation of the correlators leads to excess power consumption in an MS.